Study Finds Teens Who Use Marijuana Heavily Have Poor Memories and Brain Abnormalities

A new study by Chicago’s Northwestern University found that teens who use marijuana heavily grow up to have poor memories and also have brain abnormalities, suggesting there could be long-term effects of heavy marijuana use.

Researchers evaluated 97 volunteers with and without some sort of a mental illness. The people surveyed said they’d used marijuana daily starting at age 16 or 17, and said they had not used other drugs.

The daily marijuana users had an abnormally shaped hippocampus, the part of the brain used in storing long-term memory, and performed about 18 percent more poorly on long-term memory tasks, the researchers reported. Previous research by the Northwestern team showed heavy pot smokers had poor short-term and working memory and abnormally shaped brain structures including the striatum, globus pallidus and thalamus.

The brain abnormalities and memory problems were observed during the individuals’ early twenties, two years after they stopped smoking marijuana. Young adults who abused cannabis as teens performed about 18 percent worse on long-term memory tests than young adults who never abused cannabis.

“The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems and to sustain our relationships with friends and family,” said senior author Dr. John Csernansky in a news release.

The study is among the first to say the hippocampus is shaped differently in heavy marijuana smokers and the different looking shape is directly related to poor long-term memory performance. Previous studies of cannabis users have shown either the oddly shaped hippocampus or poor long-term memory but none have linked them.

“Both our recent studies link the chronic use of marijuana during adolescence to these differences in the shape of brain regions that are critical to memory and that appear to last for at least a few years after people stop using it,” added lead study author Matthew Smith, also in a news release.

The longer the individuals were chronically using marijuana, the more abnormal the shape of their hippocampus, the study reports. The findings suggest that these regions related to memory may be more susceptible to the effects of the drug the longer the abuse occurs. The abnormal shape likely reflects damage to the hippocampus and could include the structure’s neurons, axons or their supportive environments, researchers concluded.

Participants took a narrative memory test in which they listened to a series of stories for about one minute, then were asked to recall as much content as possible 20 to 30 minutes later. The test assessed their ability to encode, store, and recall details from the stories.
The study also found that young adults with schizophrenia who abused cannabis as teens performed about 26 percent more poorly on the memory tests than young adults with schizophrenia who never abused cannabis.

In the U.S., marijuana is the most commonly used illicit drug, and young adults have the highest — and growing — prevalence of use.

The study, titled “Cannabis-related episodic memory deficits and hippocampal morphological differences in healthy individuals and schizophrenia subjects,” was published in the journalHippocampus.